1. Field of the Invention
The present invention relates to a photoengraving process used in a method for manufacturing a semiconductor device. More particularly, the present invention relates to a method for forming a desired photoresist pattern in photoengraving process.
2. Description of the Prior Art
The photograving process is well known as a method for forming an impurity region of a desired pattern on the surface of a semiconductor substrate. Such a photoengraving process includes steps of forming a photoresist layer on a semiconductor substrate (on SiO.sub.2 layer) consisting of a semiconductor layer and a silicon oxide layer, covering the photoresist layer with a mask having a pattern corresponding to the pattern of the impurity region to be formed, and radiating the photoresist layer of the substrate with light by an aligner through the mask. When the photoresist layer is developed later, only the exposed part of the photoresist layer dissolves (in the case of a positive resist) or the non-exposed part of the photoresist layer dissolves (in the case of a negative resist), thus forming a photoresist region of desired pattern. Using this photoresist region as a mask, the silicon oxide film exposed therethrough is etched by hydrofluoric acid or the like to thereby expose the semiconductor layer. An impurity is then doped into the exposed semiconductor layer by ion diffusion or the like. In this manner, a semiconductor substrate having an impurity region of desired pattern is obtained.
However, defects are caused during masking in the photoengraving process or during covering the photoresist layer with the mask. Mechanism of formation of such defects is significantly related with the exposing method of the aligner and the brittleness of the photoresist layer. Formation of defects is especially notable when a positive resist of phenol novolak type is subjected to contact exposure (exposure is performed with the mask and the resist layer being in contact with each other). This has presented restrictions on the micronization of semiconductor elements, high packaging density, and patterning reliability.
In order to prevent formation of resist defects attributable to the brittleness of the photoresist layer, it has recently been proposed to coat the photoresist layer with a material having the property of an overcoating agent such as polyvinyl alcohol (PVA) or hydroxypropyl cellulose (HPC). Both PVA and HPC are soluble in water and alkalis and are relatively effective for preventing formation of the resist defects as described above. However, this method has not been put into practice due to the drawbacks to be described later.
When the photoresist layer is coated with PVA, (1) nitrogen gas produced by exposure is trapped between the PVA and the resist layer since PVA does not allow easy permeation of nitrogen gas. (2) For this reason, patterning becomes irregular. (3) PVA generally contains sodium which adversely affects the operation of the semiconductor element. (4) If PVA is stored in a bottle, the PVA may become turbid or gelled by the action of bacteria. When such PVA is coated, the coating characteristics are degraded and light dispersion occurs.
On the other hand, if HPC is coated on the photoresist layer, (1) HPC easily reacts with the photoresist layer upon exposure. (2) For this reason, the edge of the resist pattern becomes non-uniform, so that dimension control of the pattern is impaired. (3) HPC is also prepared using sodium as a catalyst as in the case of PVA and thus contains a relatively great amount of sodium. This makes the HPC high in cost.